Lifestyle interventions have had established efficacy for over a decade but are still not widely disseminated, largely due to high cost and patient and provider burden. Online social networks are an alternative way to deliver lifestyle counseling and delivery via this modality may virtually eliminate patient visits, the main source of cost and burden in traditional modalities. Interactions in online social networks are frequent, brief, and asynchronous because users login to their online communities during downtime during work and leisure time, or when they simply feel a need for social connection. As such, social media becomes embedded into people's daily lives. By using them we have a means to embed health behavior change programming into people's daily lives. Thus far in the literature, existing online social networks have been used as component of web- or mobile app-based lifestyle interventions but not as the primary modality for intervention delivery. The purpose of the present study is to compare a lifestyle intervention delivered entirely via private groups on the online social network Twitter to a traditional in-person group-based lifestyle intervention. We performed three pilot studies of social network-delivered interventions using Twitter and Facebook and found that this approach was both feasible and acceptable. Using a randomized trial (N=328), we will test whether a lifestyle intervention delivered via an online social network (Get Social condition) will result in a mean percent weight loss at 12 months that is not appreciably worse than the gold-standard in-person group-based lifestyle intervention (Traditional condition), i.e., the social network arm will not lose on average 2 percent less than the in-person arm. Secondary non-inferiority outcomes include weight loss at 18 months, and dietary intake and physical activity at 12- and 18-months. We also hypothesize that the Get Social condition will be less expensive than the Traditional condition. To understand for whom an online social network modality is most suited, we will test predictors of weight loss in the Get Social condition including engagement, age, sociability, neuroticism, openness, and smartphone and social network use. We hypothesize that people who are younger, more sociable, engage more on the social network, higher in neuroticism/openness, and heavier overall smartphone and social network users will lose more weight in the Get Social condition. Findings from this study may support an intervention delivery modality that is more conducive to settings like worksites, health plans, and clinics that serve large populations but have limited space, staffing, and resources for traditional in person interventions. If this efficacy trial is successful, we pla an effectiveness trial in a worksite setting to build upon our previous worksite interventions.